There are over 3.2 - 4.7 million people in the US suffering from dry eye syndrome (DES) by middle age. DES is a common multi-factorial disease of the tears and ocular surface that results in symptoms of discomfort, visual disturbance, and tear film instability with potential damage to the ocular surface. The tear system of the human eye is highly regulated and complex. Failure of any aspect will compromise the integrity of the system, resulting DES, which includes burning, foreign body sensation, reflex tearing, and visual distortion. In its most severe forms, dry eye may lead to painful recurrent corneal erosion or to secondary infection causing severe visual loss. For effective dry eye diagnosis and treatment, it is critically important to quantify the tear film thickness and its time dependent changing process. The ocular tear film is composed of three layers namely a hydrophilic mucus layer (0.02 to 0.05 5m thick), an aqueous layer (bulk of the tear film of 3 to 7 5m thick), and a lipid layer (0.1 to 0.2 5m thick). Currently, there is no suitable measurement technique that can quantify the tear film thickness in a live eye with good resolution. This proposed research is to explore the optical reflectometry technique for live eye tear film evaluation and to construct a working prototype system for phantom study to validate the proposed concept. Human tear film evaluation will also be performed. The optical reflectometry technique has been successfully used for dielectric film thickness evaluation in semiconductor industries. It however has not been used for the tear film evaluation in a live eye. With the capability of nm high resolution measurement and with thickness measurement range of 0.02 5m to 50 5m, we believe the proposed technique when developed will have significant impact to the dry eye diagnosis and treatment. The technology will provide a valuable tool for the study of the mechanisms and treatment of DES. Our collaborating team have experience in optical reflectometry, optical coherent tomography, and dry eye diagnosis and treatment. The specific aims of the proposed research are: 1. Develop a reflectometer based ultra-high resolution technology with the capability of measuring a thin film thickness of as thin as tens of nanometers that is suitable to measure the thickness of not only the tear film but also its various components. 2. Test and calibrate the technology on phantoms of tear film. 3. Test and calibrate the technology on human eyes (healthy eyes and dry eyes). PUBLIC HEALTH RELEVANCE: The proposed research will develop a powerful tool that can measure the tear film thickness without physical contact and be capable of integrating with the optical coherent tomography instrument. The measurement technique will be validated through phantom study and human eye tear film evaluation. The development will be valuable to dry eye diagnosis and treatment.